1. Field of the Invention
The invention relates to an apparatus and method for use in the measurement of fluids used in the field of oil and gas recovery. More particularly, this invention relates to an apparatus such as a mold adapted to detect the expansion or shrinkage of cement as the cement is exposed to simulated down-hole conditions, such as high-pressure, high-temperature applications. A method for measuring the expansion or shrinkage of the cement is also disclosed.
2. Description of the Related Art
It is a fact that cement will undergo chemical shrinkage when it sets. The chemical shrinkage—or the hydration volume reduction (HVR)—of a cement slurry is a direct result of water chemically reacting with the cement clinker crystalline materials forming calcium silicate crystals. The chemical shrinkage of the slurry can be substantial, e.g. seven percent of its original volume, depending on the slurry formulation. The matrix volume change of the cement slurry can lead to poor cement bonding or to the creation of micro-annuli. These micro-channels or poorly bonded areas may allow well fluids like gas, oil, steam, water, and/or the combination of fluids to migrate to the surface or to other zones of the well. The migration of well fluids will result in lower production revenue. Additionally, the migration of well fluids can increase well maintenance costs, increase casing corrosion, and reduce the life of the well. The formation of micro-annuli may require a secondary cement job or squeeze cementing, thus increasing the total well cost. The migration of well fluids can also lead to complete loss of the well due to blowout. In injection wells, the poor cement bonding can lead to a higher injection cost and lower well efficiency. These well fluids can migrate, contaminate, and pollute fresh water aquifers.
It is uncommon for oil and gas well service companies to evaluate, test or measure this important cement property. This is due primarily to a lack of the appropriate efficient and accurate equipment to continuously measure the cement shrinkage or expansion while it is curing under humid environment and under down-hole temperature and pressure.
Few methods and ideas have been tested to accurately measure volume change of cement slurry. M. E. Chenevert and B. K. Shrestha from the University of Texas have written a SPE paper on the Chemical Shrinkage Properties of Oilfield Cement. The apparatus used in the experiment consists of a high-pressure cell, a high-pressure injection pump with pressure transducer and digital gauge, a heating jacket, a vacuum pump, autoclave high-pressure tubing and valves, and an electronic thermostat with a sensing thermocouple. For each test, a thin-walled lead tube with a 1.375-inch diameter by 4 inches long is used. The lead tubes containing the slurry is capped and placed inside the high-pressure cell and inside the heating jacket. The high-pressure injection pump is used to inject mineral oil around the test sample and to the control pressure during the test. The desired heat is applied to the test sample with the heating jacket and controlled with the thermostat.
The volumetric property of the slurry is measured directly by the volume of mineral oil injected. Therefore, it is necessary to calibrate the pump and establish the relationship between the volume of oil injected to the positive-displacement pump read-out. During a test, if the temperature of the test cell changes then the pressure will change too. The behavior is due to the thermal expansion or contraction of the mineral oil. Depending on the temperature change, the pressure regulator will activate the pump by removing or adding mineral oil to the system. These can indicate a false reading of cement volumetric changes. Therefore, it is necessary to establish a mineral oil thermal expansion coefficient for each test temperature. The mineral oil correction factor will be used in slurry volumetric calculations.
Other authors, Reza Ghofrani and Heiko Plack, wrote SPE/IADC 25697 (“CaO-and/or MgO-Swelling Cements: A Key for Providing a Better Annular Sealing?”). In their experiment, they use a different apparatus to measure the matrix volume change in cement slurry. The apparatus consists of a test cell, a gear mechanism, a floating piston, potentiometer, autoclave and chart recorder. The test cell is divided into three-sections: 1) a water reservoir; 2) a sample chamber; and 3) a measuring head. A sintered metal disk and a filter paper are mounted at the bottom of the cement sample. Below the metal disk is a cavity filled with water. A metal plug with sealed assembly is used to cap the bottom of the water chamber. The autoclave curing pressure is transmitted to the cement during the test by the way of the metal plug to the water and to the cement. This will allow the cement to absorb additional water as required during the hydration process. On the top of the cement sample is a rubber cup sleeve that seals the cement against the pressurizing fluid (mineral oil). A floating piston is installed on top of the rubber cup sleeve. The axial motion of the piston caused by the changes in the matrix volume is converted to a rotational motion by a gear mechanism. The gear mechanism will alter the position of the pointer across the potentiometer resistor. The shifting position of the pointer across the resistor provides a variation of the voltage signal. The measured voltages along with the autoclave-temperature and pressure are monitored continuously by means of a chart recorder. The matrix volume change is calculated using the calibration chart. The calibration chart is expressed as piston height versus voltage signal reading.
One advantage of the Ghofrani/Plack apparatus is that fresh water is pushed into the cement matrix while curing. The test data from the apparatus can be subjective due to the measuring head design. The rubber cup sleeve that seals the cement against the mineral oil can absorb some of the displacement movement or it can swell to provide a positive movement for the piston. The mechanical tolerances on the gear mechanism can affect equipment performance from one unit to the other. The volumetric change in some slurries may be in the micro-inches spectrum and converting it to axial motion by the piston, to a rotational movement by the gear mechanism, and to electrical signal by the potentiometer. The measuring head design may be impractical for field application accuracy may be sacrifice. Currently, none of these concepts has been built commercially to support the oil well industry.